To remove the paint from an aircraft, a fiberglass boat or the like, so that it can be repainted as needed, a selective abrasion system is both desirable and necessary. Such system must have the capability of removing a paint coating without damaging the underlying metal or other substrate. The removal of paint by conventional sand blasting can result in too much anchor pattern (surface roughness) in the aluminum sheet. Blast particles such as crushed walnut shells and plastic buttons have been tried, and although brittle paint was removed, the particles are so resilient that they will bounce off of a flexible urethane coating. Agricultural products such as rice hulls and corn cob grit also have been tried, however these particles are so small and sharp that the aluminum is cut too deep. Problems in obtaining sufficient flow of these types of abrasive particles are almost insurmountable. Some agricultural abrasives contain oil so as to present a fire or explosive hazard, and leave an oil film that can prevent good paint adhesion. Thus the need for an effective selective abrasion system has persisted, particularly in view of the fact that stripping and repainting of certain large commercial aircraft can cost several hundred thousand dollars. Of course any paint removal scheme that also removes a significant amount of the metal must be avoided for safety reasons.
Applicants have therefore sought an abrasive compatible with a wet blast stripping system that is sharp, dense and hard enough to cut through and remove paint without damaging the underlying aluminum, fiberglass or a carbon fiber laminate. Their investigations have revealed that an abrasive particle must be used that has a scratch hardness characteristic not substantially greater, and preferably slightly less than the scratch hardness of aluminum, which is about 3 on the Mohs scale. It has been discovered that sodium bicarbonate is an extraordinarily good abrasive material for the foregoing application. Sodium bicarbonate has a Mohs hardness of about 3, a density similar to that of conventional blast particles such as sand, and good mass. This material is relatively inexpensive, readily available in large quantities, and in various particle sizes.
Tests of a wet blast cleaning system demonstrated that water pressures in the range of 1500-2000 psi with air pressure of 60 psi, gave satisfactory performance. However, the flow of sodium bicarbonate particles from the abrasive hopper was somewhat irregular and inconsistent, so that the process could be considered to be impractical except in a laboratory test environment. Thus applicants sought and found a solution to this problem, which is the subject of the present invention.
The general object of the present invention is to provide a new and improved abrasive feed system in a wet or dry blast cleaning process that allows use of an abrasive such as sodium bicarbonate.
Another object of the present invention is to provide a new and improved liquid-propelled abrasive cleaning system that provides selective abrasion using sodium bicarbonate particles as the abrasive material.